Discs such as magneto-optical discs or optical discs have heretofore been used as recording media of informational signals. A disc 101, as shown in FIG. 1, comprises a disc-shaped disc substrate made of a material having a light transmitting ability and a signal recording layer which is formed on at least one side of the disc substrate. When recording and/or reproducing informational signals on the disc 101, the disc 101 is loaded in a recording and/or reproducing apparatus. The disc 101 is supported at the center thereof by a disc rotating drive mechanism 102 disposed in the recording and/or reproducing apparatus and is driven to rotate around its center by the same mechanism 102. Accordingly, the signal recording layer is formed on a peripheral area which is located outside of the portion supported by the disc rotating drive mechanism 102.
Writing to the disc 101 or reading from on the disc 101 is carried out by an optical pick-up device 103 disposed in the recording and/or reproducing apparatus. The optical pick up device 103 comprises a light source and an objective lens 104 disposed at a point on an optical axis for condensing a light beam emitted from the light source so that the light beam from the light source is converged upon the signal recording portion of the disc 101. In other words, the light beam which is emitted from the light source and is impinged upon the signal recording portion of the disc 101 via the objective lens 104 is controlled in such a manner that it is constantly focussed upon the signal recording portion of the disc 101.
When an informational signal is recorded on a magneto-optical disc having a signal recording layer made of a magneto-optical material, an external magnetic field generating device is needed to apply an external magnetic field to the signal recording layer in addition to the optical pick-up device.
The optical pick up device 103 is provided with an objective lens drive mechanism 109 for moving the objective lens 104 in two directions including an axial direction and a direction perpendicular to the axial direction so that the light beam emitted from the light source is constantly focussed upon the signal recording portion of the disc 101 even when the surface of the disc 101 is oscillated or the disc 101 is eccentrically moved due to the rotation by the disc rotating drive mechanism 102. The objective lens drive mechanism 109 supports the objective lens 104 on a supporting member (not shown) in such a manner that the objective lens 104 is movable in an optically axial direction and in a direction perpendicular to the optical axis. In addition, the objective lens drive mechanism 109 drives the objective lens 104 along an optical axis and in a direction perpendicular to the optical axis by means of an electromagnetic actuator.
To protect the disc 101 from being contaminated with dust or being damaged from handling the disc 101 is housed in a disc cartridge 108 as shown in FIGS. 1 and 2 which is formed as a box-shaped cartridge main body 105. The disc cartridge 108 houses the disc 101 so that the disc 101 can be rotated within the cartridge main body 105. The cartridge main body 105 which houses the disc 101 therein is provided on at least one main side thereof with a recording and/or reproducing opening 106 through which at least a part of a signal recording area of the disc 101 is exposed in a radial direction, and a chucking opening 107 through which the central portion of the disc 101 formed with a center hole is exposed. Accordingly, the disc 101 housed in the disc cartridge 108 is supported from the center hole and is rotated by the disc rotating drive mechanism 102 through the chucking opening 107. Writing and/or reading of informational signals is carried out by the optical pick-up device 103 which faces to the disc 101 through the recording and/or reproducing opening 106.
The recording and/or reproducing opening 106 which is formed on the cartridge main body 105 is closed by a shutter member (not shown) that protects the disc 101 when not in use. The shutter member has a shutter portion which is formed to close at least the recording and/or reproducing opening 106 and is movable in such a direction that it opens and closes the recording and/or reproducing opening 106, and which is supported on the cartridge main body 105. Accordingly, the shutter member is moved to a position in which it closes the recording and/or reproducing opening 106 when the disc cartridge 108 is not in use while it is moved by a shutter opening mechanism to an open position when the disc cartridge 108 is loaded in the recording and/or reproducing apparatus.
In the above mentioned recording and/or reproducing apparatus, the distance between the objective lens 104 and a first surface of disc 101 faced by the objective lens 104 i.e. the working distance, is determined by the optical characteristics of the objective lens 104 of the optical pick up device. In order to make the density of the informational signals which are written to and/or read from the signal recording layer higher, it is necessary to converge a light beam B on the signal recording layer as a smaller spot. At this end, the working distance is made as short as possible so long as no problem arises in the design and manufacture of the objective lens 104.
Since the working distance is made as short as possible, the objective lens 104 must be brought as close as possible to the disc 101. As shown in FIG. 1, the objective lens drive mechanism 109 has a diameter which is larger than the outer diameter of the objective lens 104.
Thus, in order to place the objective lens drive mechanism 109 within the cartridge main body 105, it is therefore necessary to form the cartridge main body 105 with a recording and or reproducing opening 106 that is large enough to enable the objective lens drive mechanism 109 to move into the cartridge main body 105.
If a larger recording and/or reproducing opening 106 is formed in order to meet the above mentioned requirements, it is also necessary to provide a larger shutter member for opening and closing the recording and/or reproducing opening 106. This makes it difficult to provide a compact disc cartridge 108 since the shutter member is movably supported by the cartridge main body 105 so that it opens and closes the recording and/or reproducing opening 106 as mentioned above. If the recording and/or reproducing opening 106 is made larger, it becomes very difficult to form a flat main side of the disc cartridge since the cartridge main body 105 is formed of a synthetic resin by a molding method such as injection molding using an injection molding machine.